code: def backward(self, indexes, gy):
      x = self.get_retained_inputs()[0]
      return (
          (2 * numpy.pi) ** -0.5
          * exponential.exp(-0.5 * x**2)
          * gy[0],
      )
  
file_path: chainer/chainer/functions/math/ndtr.py
imports: import math
  import warnings
  import numpy
  import chainer
  from chainer.backends import cuda
  from chainer import function_node
  from chainer.functions.math import exponential
  from chainer import utils
  from chainer.utils import type_check
label_desc: Use image node_id 5 for calling the Ndtr obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: unknown